Airbag devices are known in which gas from an inflator is supplied through a gas supply pipe fixed to a seatback to a bag attached to the gas supply pipe, and the bag is inflated so as to surround an occupant from the front and the side in a collision (see Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-344044). Airbag devices are also known in which head airbags protrude out from left and right end portions of a seat headrest in a forward direction, and join up in front of the head of an occupant, and an auxiliary head airbag protrudes out in a forward direction from a central portion of the seat headrest and joins up with the pair of head airbags (see Patent Document 2: JP-A No. 2013-018378).